UNDERWATERSURVEY 123
SURVEY USING TAPE-MEASURES, GRIDS
AND DRAFTING FILM
With control points installed on the site, it is now pos-
sible to collect measurements that can be used to calculate
the positions of the points. Each point on a site has a posi-
tion given in three dimensions: xand yhorizontally and
zfor depth or height. Positioning under water is largely
a matter of measuring distances between objects. The
most typical tools used for this are the tape-measure, the
drawing frame and the rigid rule.
A small drawing board called a slate or clipboard is most
often used for recording measurements under water. The
slate should be slightly larger than A4 size. Plastic sheet
of 3 –5 mm thickness is usually sufficiently rigid. The slate
should sink rather than float so, if necessary, a small strip
of lead sheet can be attached to the bottom. It should have
a short lanyard with a clip to attach it to the diver. If a
slate is to be carried in a net bag, then the corners should
be rounded. When carried in a bag with other equipment,
the danger of figures smudging must be guarded against.
A pencil should be attached to the slate with a lanyard long
enough to allow it to be used comfortably anywhere on
the slate. Propelling pencils made of plastic work well under
water (soft (2B) leads are recommended). Ordinary pen-
cils break easily when waterlogged.
The measurements are written on a sheet of plastic or
drafting film attached to the slate with clips or electrical
tape (plate 14.2). Standard waterproof forms can be made
by photocopying a paper form on to drawing film
(75 micron plastic paper will pass through most photo-
copiers and printers). Each form should have space for
the site name, the site code, the date, a dive reference
number, diver’s name, estimated visibility and current,
plus room for a small sketch. Numerous different types
of form can be created – for distance measurements, off-
set measurements, depth measurements, blank forms for
sketches and forms with square grids for drawing frames.
Write clearly in capitals and avoid smudging work. Use
standard units and conventions, which should be agreed
beforehand. Any recording forms that have measure-
ments on them should be well looked after. The forms are
the primary record of any work and should be kept even
after they have been processed. Keeping a notebook with
day-to-day accounts, speculations and ideas about the site
is often useful, as the notes can be helpful later when the
measurements are being processed.
Many different varieties of tape-measure are available
with different materials, lengths and designs. A tape-
measure suitable for survey work under water is no more
than 30 m (98 ft) long and has graduations as fine as
the site-plan requires. Open-frame tapes are better as
they can be easily rinsed after use and dismantled to
remove silt and grit. Yellow tapes look good in pho-
tographs, whereas white tapes tend to flare in bright
light and are harder to read.
The most common type of tape-measure is made of
glass fibre-reinforced plastic and those with stainless steel
or plastic fittings will last the longest. Fibreglass tapes
are cheap and readily available from builders’ merchants
but they do stretch with use. Steel-cored tapes look very
similar but stretch very little, although they are slightly
more expensive and can kink if not handled properly.
A typical steel-cored tape is accurate to about 6 mm at
30 m in ideal conditions whereas a fibreglass tape is only
accurate to 30 mm at 30 m. The achievable accuracy does
depend on the conditions under water, as any water-
movement will tend to make the measurement less accu-
rate. Measurements over 30 m are not recommended
as this length of tape is difficult to manage and sag in the
tape makes the measurements insufficiently accurate.
Given that fibreglass tapes will stretch, the correct
amount of pull has to be used to ensure the correct dis-
tance is recorded. The weight of the tape itself tends to
make it sag so measurements will tend to be slightly long;
in pulling the tape straight the diver may end up pulling
it too much and record a short measurement. Any water
movement will make the tape bow outwards and this
has the same effect as sagging, so where possible take
measurements at slack water. It is possible to measure the
correct tension but this is difficult to do under water and
is very time-consuming. Getting the correct tension
can only come with practice, but it is more important
to be consistent with the amount of tension used. It is
important to train divers in the use of these techniques
on land before they use them under water.
The free (zero) end of the tape should be hooked on
to a control-point and unwound in the direction of the
point to be measured. Pull the tape tight over the point
to be measured and record the distance at the centre of
that point. The names of the relevant points should be
clearly recorded on the form along with the measurement
itself. Care is needed here, as a mistake can be made when
the measurement is written down and when it is read by
whoever is processing the measurements. The results
should be recorded on a form clearly enough for some-
one who has never visited the site to be able to process
the results. It is often necessary to refer back to the
recording forms during processing to help decide whether
a measurement should be rejected. A number of factors
affect the quality of measurements: working in low vis-
ibility, strong currents and deep water all tend to produce
more frequent mistakes.
Tapes are prone to being snagged on other objects
between the two points being measured. Where both ends
of the tape cannot be seen the tape should be checked for
snags before the measurement is taken. Where practical,